Sexually transmitted infections (STIs) affect 340 million new people each year worldwide, including an estimated 20 million new infections in the US, adding an estimated $17 billion to US national healthcare costs per year. This application describes a multi-PI translational research project with the goal of producing new methods for prevention and treatment of genital herpes simplex virus (HSV-2) infections. In work that over the past 25 years, one of the PIs (Saltzman) has shown: 1) antibodies, DNA, RNA, and peptide nucleic acids can be released slowly from biocompatible polymers, including nanoparticles (NPs), and these agents are active at mucosal surfaces; 2) surface-modified NPs penetrate readily through unstirred human cervical mucus; 3) vaginal rings releasing protein antigens can stimulate long-lasting, mucosal immunity; 4) polymer NPs loaded with siRNA delivered to the vaginal mucosal surface are capable of knocking down genes that interfere with HSV transmission in mice; and 5) NPs loaded with siRNA directed against SIV genes reduce viral loads in chronically infected macaques. The second PI (Iwasaki) has done pioneering work in understanding the immune responses to and pathogenesis of genital HSV-2 infection. In this application, we propose to exploit the experience we have gained in past studies, by presenting a focused plan to develop a safe and effective topically- administered NP delivery system for prevention and treatment of HSV-2 infections. An important goal of the proposed work is to produce preclinical data that provides a clear route for translation to clinical practice. We will accomplish this goal in three specific aims: First, we will optimize multifunctional NPs for topical siRNA delivery in the female reproductive tract. The multifunctional NPs will be based on our biocompatible PLGA delivery systems with additions to improve effectiveness and safety: cationic polymers will enhance siRNA loading and endosomal escape; PEG will facilitate mucus penetration; and shedding of PEG will facilitate vaginal retention and cellular uptake. We will test efficiency, duration, and toxicity of these multifunctional NPs after vaginal administration in mice. We will also examine the toxicity of our best preparations in non- human primates. Second, we will determine the effectiveness of these multifunctional NPs for prevention of HSV-2 infections. Third, we will measure the effectiveness of these NPs for treatment of recurrent HSV-2 infections. These studies will use state-of-the-art animal models to test a new method for prevention of HSV-2 transmission and treatment of recurrence.